There are many design situations in which a rotatable shaft must pass through a wall structure into a fluid medium. Often the aperture in the wall structure through which the shaft passes must be sealed in order to retain the fluid on one side of the wall. One such situation is a propeller drive shaft extending from the interior of the hull of a vessel to the exterior, passing through an aperture in the hull. The aperture must be sealed to prevent water from entering the vessel. Several devices for sealing such apertures have been developed and are discussed below.
Stuffing boxes generally consist of a bronze or other metallic housing or plate which is fastened over the aperture in the hull of the vessel and includes a hollow, cylindrical tube which extends into the vessel. The propeller drive shaft is received in the cylindrical tube, and passes through the housing and through the aperture. In most applications for pleasure craft, stuffing boxes include a metallic nut, which can be threadingly received in the cylindrical tube, inserted on the propeller shaft. A sealing material or gland, such as packing rings, which can be wax or graphite impregnated braided flax rope is provided. The packing rings are cut and placed around the propeller drive shaft between the metallic nut and inside the cylindrical tube. The brass nut is then threaded into the cylindrical tube and tightened against the packing rings, compressing the packing against the propeller drive shaft in an attempt to create an essentially watertight seal.
When stuffing boxes are used on commercial vessels having large diameter propeller drive shafts, the packing is contained within the body of the stuffing box. Instead of the metallic nut used with pleasure craft, as described above, a housing cover or cap is provided to compress the packing. Threaded fasteners extend through the cover or cap and are received in threaded bores in the stuffing box.
Stuffing boxes have numerous problems. Packing seals against the rotating shaft and causes wear damage to the shaft. Many problems with stuffing boxes are associated with the fact that they need constant adjustment. When the propeller shaft is operating, the packing gland should be somewhat loose to permit adequate amounts of water to enter the stuffing box to moisten the packing and cool and lubricate the rotating shaft. If no water enters there is no lubrication and the friction between the rotating shaft and the packing increases, causing the shaft to become hot. This damages the packing and makes the shaft more susceptible to being scored, and the shaft may have to be repaired or replaced.
After operation and before the vessel is docked, the stuffing box should be tightened to keep all water from entering the vessel. Due to the difficulty and inconvenience in accessing the stuffing box, most vessel operators do not adjust it as frequently as they should. Therefore, excess water often enters the vessel when it is docked, because the stuffing box may still be loose, and a bilge pump is used to pump the water outboard. Bilge pumps can fail or malfunction, however, and most inboard water damage to vessels occurs because water enters through the packing gland, and the bilge pump subsequently malfunctions.
Another major problem with stuffing boxes is that they can leak, even when adjusted properly. Compressed flax is not impervious to water. Another problem with stuffing boxes is that there is very little flexibility transverse to the axis of the shaft in a packing gland. As the vessel is driven backward and forward, the shaft moves back and forth and vibrates or wobbles as it rotates. The movement of the shaft eventually creates gaps in the packing rings, and water leaks through the gaps.
Another problem with stuffing boxes is that the packing wears out quickly due to the normal rotation of the shaft. As it wears, more water enters the vessel causing expensive vessel maintenance, both in the form of repairs to the vessel and frequent replacement of the packing. In this respect, when the packing is replaced, the vessel may need to be lifted out of the water which is an expensive, time-consuming operation. Even when used and maintained properly, stuffing boxes need to be repacked about every six months to two years, depending on hours of use and operating conditions. To properly repack a stuffing box in some vessels, the back of the vessel may have to be removed from the water. This is expensive and inconvenient. In some situations, as where air-sealed stuffing boxes from Duramax, Inc. are employed, the vessel need not be removed from the water.
Another problem with stuffing boxes is that if the nut is too tight, the heat due to friction can increase to the point where the nut heats up and "freezes" to the shaft. If this happens, the stuffing box can be ripped free of the hull aperture, flooding the vessel.
A additional problem with stuffing boxes is that the housing is dimensioned for one specific vessel hull aperture configuration. Stuffing boxes therefore cannot be easily retrofitted into vessels with hull aperture configurations different than that for which the stuffing box was designed.
Another problem with stuffing boxes is that they cannot be adjusted to align with the axis of the shaft. Therefore, the shaft is often not aligned with the cylindrical tube on the stuffing box through which it passes, making it difficult to obtain a tight seal and to avoid damage to the shaft.
Additionally, the water which enters a vessel must be pumped outboard. Water entering the bilge through the stuffing box mixes with oil and other contaminants in the bottom of the vessel. The bilge pump pumps the water, mixed with the contaminants, into the surrounding water. The Federal Water Pollution Control Act prohibits discharge of oil or oily waste into or upon navigable waters of the United States or the waters of the contiguous zone if such discharge causes a film or sheen upon or a discoloration of the surface water.
To alleviate the problems with stuffing boxes, mechanical seals were developed. Mechanical seals need no repacking, little maintenance and, if properly selected and installed, virtually eliminate water from entering the bilge. The life span of a mechanical shaft seal if correctly adjusted is approximately 10,000 to 15,000 operating hours. Putting this into perspective, a busy season for a pleasure vessel is 600 hours and a commercial vessel operates approximately 2,000 to 5,000 hours per year. Therefore, this equates to a minimum of two years and a maximum of twenty-five years of use. The most common mechanical seals are the "lip" seal and "face" or "surface" seal.
The lip seal is a flexible, stationary annular seal, usually made from rubber, which surrounds and fits tightly against the propeller drive shaft creating a seal while allowing rotation of the shaft. Lip seals have a number of problems when used in marine applications. First, lip seals tend to wear grooves in the propeller drive shaft, which is usually made of stainless steel. Once a groove is worn into the steel, water tends to leak past the groove. Further, the groove creates a weak point in the shaft. The shaft must then be repaired by welding or replaced. Second, any shaft misalignment or cross-axial shaft vibration causes leakage because the lip seal is not flexible enough to compensate for a shaft not centered within the seal. Third, the rubber in the lip seal must remain supple in order to conform to the surface of the shaft. The rubber seal is exposed, however, to a harsh environment of salt water, oil, hot and cold temperatures, and loses its softness quickly, leading to water leakage.
The most popular mechanical seal used for rotating propeller shafts is the face or surface seal. The face seal comprises two finely machined sealing surfaces pressed together to form a watertight seal. In marine applications, the sealing faces or surfaces are typically facing, axially-perpendicular flat sides of two annular rings, one of which is mounted on a rotating shaft, such as a propeller drive shaft. In this respect, each ring has an inner diameter defining a center bore or opening dimensioned such that the rings can be slidingly mounted onto the propeller drive shaft. (Optionally, either ring can be provided in two pieces and fastened onto the shaft whereby the pieces are joined by compression screws.) Each ring also has an outer diameter, and a flat annular sealing surface defined between the inner and outer diameters. One of the two rings is called the seal ring and is fixed to the propeller shaft and, therefore, rotates with the shaft. The other ring is called a friction ring, which is typically stationary and fastened to a housing through which the propeller shaft passes. The annular sealing surface of the seal ring mates with the annular sealing surface of the friction ring, creating a watertight seal. In standard face or surface seal devices, a loading or biasing means such as a spring or rubber bellows biases the friction ring and seal ring together and provides a compression force that helps create and maintain the seal.
A major advantage of a face seal is that it does not seal against the propeller shaft. As opposed to a lip seal or to a stuffing box, therefore, face or surface seals allow the shaft to rotate with less resistance, resulting in better performance and improved fuel economy. Additionally, the seal will not wear the propeller shaft.
One common type of surface seal uses a carbon-graphite friction ring and a stainless steel sealing ring. Although these materials usually function well, there are problems associated with them. The carbon-graphite friction ring is brittle, making it susceptible to cracking, and it is easily scratched or pitted, which can lead to leakage between the sealing surfaces. Further, carbon graphite and stainless steel are dissimilar materials and the contact between the two can lead to crevice corrosion and degradation of the seal face. As it will be understood, any imperfections in the friction ring caused by corrosion of the surface or by scratches in the surface will cause leakage at the seal face.
Another problem which occurs with the stainless steel seal ring is that the ring may be secured to the shaft with set screws. The set screws are usually stainless steel and electrically connect the set screw seal face to the shaft, promoting electrolysis on the seal face.
Another problem with surface seals is that maintaining proper compression between the two seals is critical. As the shaft moves in the axial direction in response to the reverse or forward thrust of the propeller, the compression between the sealing surfaces varies. Too much pressure between the seal ring and friction ring causes undue wear of the seal, and too little pressure allow leakage between the seals.
It is known to use a convoluted neoprene rubber bellows to load or bias the friction ring towards the seal ring to maintain pressure between the two. The biasing force of the rubber bellows is largely dependent on the durometer of the rubber. As the rubber is exposed to the environment and ages, it loses its elasticity and the amount of tension on the seal face decreases. This results in leakage and the need to adjust the position of the seal ring on the shift in order to compress the bellows more to increase the compressive force between the seal ring and friction ring.
A wire-reinforced rubber bellows surface seal device was introduced in the late 1980's. It is easy to install and remove, and it maintains a generally constant pressure between the seal ring and friction ring, thereby extending the life of the seal. The wire-reinforced rubber bellows seal device is comprised of three main components: a rigid stern tube, a rigid friction ring and a flexible hose connecting the two. The center of the hose is a bellows structure reinforced with a stainless steel coil spring. One end of the hose is clamped over the stern tube and the other end is clamped over the friction ring. The propeller shaft passes through the friction ring, the flexible hose and the stern tube. The seal ring is fixedly attached to the shaft, adjacent the friction ring. The rubber bellows and spring bias the friction ring towards the seal ring, thereby maintaining relatively constant pressure between the two. The spring eliminates the loss of compressive force associated with degradation of the rubber.
It has been known to make the friction ring of the wire-reinforced bellows of high impact, high temperature, oil-impregnated cast nylon. This material is extremely impact resistant, can withstand heat of approximately 350.degree. to 400.degree. F. and has a very low water absorption rate. Therefore, it will not crack like the brittle carbon-graphite friction rings. Further, because it is plastic, the problems associated with carbon-graphite seals such as electrolysis and crevice corrosion are eliminated. In addition, the seal ring can be electrically insulated from the vessel. Compression screws fasten the shaft clamp to the seal ring. The shaft clamp can compress a rubber or plastic O-ring against the seal ring thereby wedging the O-ring between the seal ring and the shaft, electrically isolating the stainless steel seal ring from the propeller shaft.
When surface seals are in use, a thin film of water should remain between the two seal faces. This thin film acts as a lubricant on the seal faces and keeps the faces cool, extending the life of the seal. On a displacement hull, bleeding off any trapped air in the seal allows water to reach the seal faces, keeping them lubricated. On a high speed hull, a vacuum is drawn in the stern tube as the speed of the vessel increases. Water can then be injected into the seal to keep the seal face lubricated. The wire-reinforced rubber bellows surface seal heretofore described has utilized an air vent/water injection fitting which can either remove air from or inject water to the sealing surfaces.
As stated above, the wire-reinforced rubber bellows surface seal was an improvement over stuffing boxes, lip seals and other surface seals. It provided a surface seal arrangement wherein the friction ring would move forward or aft by means of the flexible tube and spring-loaded bellows, in response to axial movement of the propeller shaft and the seal ring fixed to the shaft. Therefore, surface pressure between the two sealing surfaces was constantly maintained without the sealing faces being excessively compressed. This seal still had a number of problems, however. First, because the rubber hose is soft and flexible, it can be moved or dislodged by a person stepping on it or by objects striking it. When the rubber hose is moved or dislodged, the seal faces are moved out of alignment, thus interfering with the sealing contact, and substantial leakage occurs. Additionally, as with all prior art seals, this seal cannot be adjusted to the axis of the propeller shaft after it is installed, nor can it be easily retrofitted onto different vessel hull/aperture configurations.
The present invention overcomes the disadvantages of the prior art by providing an adjustable surface or face-sealing device, easily adaptable to most hull aperture configurations, which can be aligned with the axis of the propeller drive shaft after it is installed, and which is constructed of rigid components so that force applied to the device will not dislodge the sealing surfaces.